The adaptive immune system has two different responses, the humoral immune response and the cellular immune response. The first is characterized by an antibody response in which these antibodies bind to surface epitopes of pathogens while the latter is characterized by cytotoxic T-lymphozytes (CTLs) that kill already infected cells. Both immune responses are further stimulated by T-helper cells that activate either the B-cells that are producing specific pathogen binding antibodies or T-cells that are directed against infected cells.
The specificity of the interaction between the antibodies produced by B-cells and the pathogen is determined by surface structures of the pathogen, so called B-cell epitopes, while the specificity of the interaction of CTLs with the infected target cell is by means of T-cell epitopes presented on surface molecules of the target cell, the so-called major histocompatibility complex class I molecules (MHC I). This type of T-cell epitopes (CTL-epitopes) are fragments of the proteins from the pathogen that are produced by the infected cell. Finally, the specificity of the interaction of the T-helper cells with the respective B-cell or CTL is determined by binding of receptor molecules of the T-helper cells to the other type of T-cell epitopes (HTL-epitopes) presented by the MHC class II molecules (MHC II) on the B-cells or CTL-cells.
Binding of the antibodies to the B-cell epitopes requires the B-cell epitope to assume a particular three-dimensional structure, the same structure that this B-cell epitope has in its native environment, i.e. when it is on the surface of the pathogen. The B-cell epitope may be composed of more than one peptide chain and is organized in a three dimensional structure by the scaffold of the protein.
The T-cell epitopes, however, do not require a particular three-dimensional structure, rather they are bound by the respective MHC I or MHC II molecule in a very specific manner. CTL epitopes are trimmed to a size of 9 amino acids in length for optimal presentation by the MHC I molecules, while HTL epitopes make a similar interaction with the MHC II molecules but may be longer than just 9 amino acids. Important in the context of this invention is, that the binding of the epitopes to the MHC molecules follows very particular rules, i.e. only peptides with specific features will be able to bind to the respective MHC molecule and hence be useful as epitopes. These features have been thoroughly investigated and from the wealth of epitopes known, prediction programs have been developed that are able to predict with high accuracy epitopes that are able to bind to the MHC molecules. Peptide strings composed of several such T-cell epitopes in a linear peptide chain are now being engineered as vaccine candidates.
In general an efficient vaccine should induce a strong humoral as well as a strong cellular immune response. It has been shown that by repetitive antigen display of B-cell epitopes a strong humoral immune response can be achieved. Virus-like particles (VLPs) can be used as an efficient tool to present B-cell epitopes in a regular, repetitive and rigid manner, and hence VLPs are now widely used for vaccine design. Another approach for repetitive antigen display has been described in Patent EP 1 594 469 B1. In this patent self-assembling peptide nanoparticles (SAPN) composed of trimeric and pentameric protein oligomerization domains have been engineered that repetitively display B-cell epitopes on their surface. The B-cell epitopes were attached at the end of the oligomerization domains in order to guarantee that the B-cell epitopes are presented at the surface of the nanoparticles in multiple copies. One of the most frequently encountered protein oligomerization motif is the coiled-coil structural motif and this motif can efficiently be used in the design of these SAPN.